880 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



Fig. 211. 



optically the curvature of the side of the bottle. As the refractive 

 indices of Sonstadt's solution and balsam are not very different, this 

 plan succeeded admirably. A thin cover-glass was similarly cemented 



to the opposite side (front face) 

 of the bottle, which was thus 

 optically flattened front and 

 back. Some^ Sonstadt's solu- 

 tion (sp. gr. 2-77) being 

 introduced, a fragment of 

 aragonite (sp. gr. 2 '9) was 

 dropped in; it at once, of 

 course, sank to the bottom. 

 Next a piece of calcite (sp. 

 gr. 2 • 7) was added ; it floated 

 on the surface. The spicules 

 lying in water, were freed as 

 far as possible from air by 

 boiling, and with the air- 

 pump. With a dipping-tube 

 the water and spicules to- 

 gether were taken up and 

 added to the top of the 

 Sonstadt's solution, where they floated. The tube was then left to stand 

 in order that diffusion might take place. After some hours the water 

 and Sonstadt's solution had become gradually mixed, giving a column 

 of fluid with a specific gravity of about 2 • 4 at the top and 2 • 77 at the 

 bottom. The calcite and the spicules floated at different levels (the 

 spicules being above) in layers of fluid having respectively the same 

 specific gravity as themselves. A fragment of pure quartz (sp. gr. 

 2 • 65), and another of adularia felspar (sp. gr. 2 • 68) were next added ; 

 the quartz sank to a level below the spicules, the felspar remained 

 above. As the contents of the tube could be easily examined under 

 the Microscope with a 1 in. or even a 1/2 in. lens (Zeiss's C), one 

 could make certain of the absence of air-bubbles, vacuoles, or other 

 troubles; and as the spicules could be seen individually, it was 

 possible to determine the specific gravity of a single one. The 

 spicules did not all lie at exactly the same level, but formed a zone 

 thickest towards the middle, and thinning off above and below ; a few 

 stragglers were seen at some distance on either side, but this was 

 owing either to adhesion to the side of the tube, or attached im- 

 purities. 



The specific gravity could now be exactly determined. Two 

 rectangular axes are ruled, fig. 212 ; on one distances are taken to 

 represent the densities of the calcite, quartz, and felspar ; on the 

 other the exact distances between the middle line of each fragment as 

 it floats in the tube are measured off. 



These distances were obtained by gumming two scales divided 

 into millimetres on the stage of the Microscope at right angles to 

 the glass slide carrying the experimental tube, i. e. parallel to this 

 tube (fig. 211, Sci and Scz). The calcite was brought into focus, and 

 the position of one edge of the glass slide read off on the scales ; the 



